Semiconductor device and method for manufacturing the same

Abstract

An object is to provide a high reliability thin film transistor using an oxide semiconductor layer which has stable electric characteristics. In the thin film transistor in which an oxide semiconductor layer is used, the amount of change in threshold voltage of the thin film transistor before and after a BT test is made to be 2 V or less, preferably 1.5 V or less, more preferably 1 V or less, whereby the semiconductor device which has high reliability and stable electric characteristics can be manufactured. In particular, in a display device which is one embodiment of the semiconductor device, a malfunction such as display unevenness due to change in threshold voltage can be reduced.

The invention claimed is: 
     
       1. A method for manufacturing a semiconductor device, comprising:
 forming a gate electrode; 
 forming a gate insulating layer over the gate electrode; 
 forming an oxide semiconductor layer containing an In—Ga—Zn—O-based oxide semiconductor over the gate insulating layer, 
 performing first heat treatment on the oxide semiconductor layer in an inert gas atmosphere to dehydrate or dehydrogenate the oxide semiconductor layer; 
 forming source and drain electrodes over the oxide semiconductor layer; 
 forming an inorganic insulating layer over the oxide semiconductor layer and the source and drain electrodes; and 
 performing second heat treatment on the inorganic insulating layer, 
 wherein the oxide semiconductor layer is crystallized after the first heat treatment, 
 wherein in the In—Ga—Zn—O-based oxide semiconductor, a content of In is larger than a content of Ga, and a content of Zn is smaller than a sum of the content of In and the content of Ga, and 
 wherein a length of the gate electrode is shorter than a length of the oxide semiconductor layer in a channel length direction. 
 
     
     
       2. The method according to  claim 1 , wherein the inert gas atmosphere is a nitrogen atmosphere or a rare gas atmosphere. 
     
     
       3. The method according to  claim 1 , wherein the first heat treatment is performed in a range of 350° C. and 750° C., inclusive. 
     
     
       4. The method according to  claim 1 , wherein the second heat treatment is performed in an air atmosphere, an oxygen atmosphere, a nitrogen atmosphere, or a rare gas atmosphere. 
     
     
       5. The method according to  claim 1 , wherein the second heat treatment is performed in a range of 100° C. and a highest temperature in the first heat treatment, inclusive. 
     
     
       6. The method according to  claim 1 , wherein the inorganic insulating layer includes at least one of silicon oxide, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, and aluminum nitride. 
     
     
       7. The method according to  claim 1 , further comprising:
 forming a channel protective layer over and in contact with the oxide semiconductor layer, before forming the source and drain electrodes. 
 
     
     
       8. The method according to  claim 1 , wherein the content of Zn is smaller than each of the content of In and the content of Ga. 
     
     
       9. A method for manufacturing a semiconductor device, comprising:
 forming a gate electrode; 
 forming a gate insulating layer over the gate electrode; 
 forming an oxide semiconductor layer containing an In—Ga—Zn—O-based oxide semiconductor over the gate insulating layer, 
 performing first heat treatment on the oxide semiconductor layer in an inert gas atmosphere to dehydrate or dehydrogenate the oxide semiconductor layer; 
 forming source and drain electrodes over the oxide semiconductor layer; 
 forming an inorganic insulating layer over the oxide semiconductor layer and the source and drain electrodes; 
 performing second heat treatment on the inorganic insulating layer; and 
 forming an electrode layer electrically connected to one of the source and drain electrodes through an opening provided in the inorganic insulating layer, 
 wherein the oxide semiconductor layer is crystallized after the first heat treatment, 
 wherein in the In—Ga—Zn—O-based oxide semiconductor, a content of In is larger than a content of Ga, and a content of Zn is smaller than a sum of the content of In and the content of Ga, 
 wherein a length of the gate electrode is shorter than a length of the oxide semiconductor layer in a channel length direction, and 
 wherein a width of one of the source and drain electrodes is shorter than a width of the oxide semiconductor layer in a channel width direction. 
 
     
     
       10. The method according to  claim 9 , wherein the inert gas atmosphere is a nitrogen atmosphere or a rare gas atmosphere. 
     
     
       11. The method according to  claim 9 , wherein the first heat treatment is performed in a range of 350° C. and 750° C., inclusive. 
     
     
       12. The method according to  claim 9 , wherein the second heat treatment is performed in an air atmosphere, an oxygen atmosphere, a nitrogen atmosphere, or a rare gas atmosphere. 
     
     
       13. The method according to  claim 9 , wherein the second heat treatment is performed in a range of 100° C. and a highest temperature in the first heat treatment, inclusive. 
     
     
       14. The method according to  claim 9 , wherein the inorganic insulating layer includes at least one of silicon oxide, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, and aluminum nitride. 
     
     
       15. The method according to  claim 9 , further comprising:
 forming a channel protective layer over and in contact with the oxide semiconductor layer, before forming the source and drain electrodes. 
 
     
     
       16. The method according to  claim 9 , wherein the content of Zn is smaller than each of the content of In and the content of Ga.